[roll third_party/pigweed] docs: Set mermaid theme to match furo

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/149852

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: a14d5d5c4d9fcba..f52264dc2aedd9a
Roller-URL: https://ci.chromium.org/b/8779536252644164113
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I7672d9ab36974e3a66b35c6078d5a9af8a603d5d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/149823
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index a14d5d5..f52264d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit a14d5d5c4d9fcba23a85b41c543ab285c5243b29
+Subproject commit f52264dc2aedd9a84373d4af0d66a8b5633e3948